Police have questioned Boris Johnson's chief of staff, Sir Simon Milton, and his chief spin-doctor, Guto Harri, as part of their investigation into the Mayor's former deputy, Ian Clement.

Officers from Scotland Yard's Specialist Crime Command entered City Hall last week and took statements from Sir Simon and Mr Harri. Documents were also seized. A number of other GLA officials, believed to include the authority's director of finance, Martin Clarke, were also interviewed.

Mr Clement, the deputy mayor for government relations, dramatically resigned three weeks ago after allegedly falsifying his official expenses. The Standard revealed that he had claimed public money for at least three meals with Tory council leaders who said they had not dined with him on the dates concerned.

On at least two of the occasions, Mr Clement was in fact entertaining his lover, 23-year-old Tory party agent Claire Dowson, whom he met while working on the Johnson campaign and has escorted at several public events.

Mr Clement also has a long-term partner and child at his main home in Dartford. He has been accused of taking Ms Dowson on a hotel stay in Europe at public expense and of misusing a corporate credit card issued to him in his former role as leader of Bexley council. Bexley councillors will tomorrow vote on a motion ordering the authority to conduct a full investigation of Mr Clement's allowances and expenses.

Sir Simon has been questioned because he has responsibility for the administrative arrangements for the expenses system.

It is not clear why Mr Harri has been interviewed. He was unavailable for comment last night.

A City Hall spokeswoman confirmed the police interviews but said the Mayor himself had not been questioned and "was not expected to be". However, it emerged last month that Mr Johnson had personally signed off some of Mr Clement's most recent expenses.

Mr Clement had been repeatedly warned, including by Mr Johnson, about the use of his GLA credit card. He used it to upgrade his ticket on a flight to the Beijing Olympics when the Mayor and other officials flew economy.

The card was confiscated three weeks before he resigned after it emerged that Mr Clement had used it for personal items, including groceries and work on his Jaguar car.

The fraud inquiry began last month after City Hall reported Mr Clement to police. The spokeswoman said Sir Simon and Mr Harri were not interviewed under caution. "We are co-operating with the inquiry," she said.